1. Field of the Invention
The present invention relates to a recording apparatus for recording an image onto a recording sheet which is conveyed through a recording unit and, more particularly, to a conveying roller for conveying the recording sheet and a bearing of the conveying roller.
2. Description of the Related Art
Generally, a recording apparatus having a function of a printer, a copying apparatus, a facsimile, or the like is constructed so as to form an image (including characters, symbols, or the like) onto a recording sheet such as paper, cloth, plastics sheet, OHP sheet, or the like by using recording head on the basis of image information. As a scanning system in the recording apparatus, there are serial and line types. The serial type is a system in which the image is recorded while alternately repeating a main scan for moving the recording head along the recording sheet and a sub-scan for conveying the recording sheet at a predetermined pitch. The line type is a system in which the image is recorded only by the conveyance (sub-scan) of the recording sheet while recording the image of one line in a lump. The recording apparatuses can be classified into an ink jet type, a thermal transfer type, a laser beam type, a thermal sensitive type, a wire dot type, and the like in accordance with the kind of recording head.
In recent years, in recording apparatuses, particularly in ink jet recording apparatuses, image quality of the output image has been improved and the precision necessary regarding the recording operation to realize high image quality is continuing to improve. For example, in ink jet recording apparatus, one means for improving the image quality of the recording image is a means for reducing the ink discharge amount per dot and decreasing the diameter of each dot on the recording sheet, to reduce granularity of the dots of the ink discharged onto the image. When the size of dot decreases, an area where the dots have to overlap enters the state where they do not overlap. In other words, if an arrival position of the dot changes slightly, the non-overlap state appears (or an area where the dots do not have to overlap enters the state where they overlap) and a concentration or hue of this area is deviated. Such a deviation in the concentration or hue becomes a white stripe, a black stripe, or an uneven color, causing a deterioration in image quality. The positional deviation between the dots mentioned here is on the level of tens of μm to a few μm. A means for assuring such a precision is necessary.
One important mechanism regarding the improvement of image quality is the mechanism for conveying the recording sheet by a plurality of conveying rollers. In such a mechanism, in order to improve the image quality, first, it is necessary to improve the eccentricity, cylindricity, and diameter tolerance of the conveying rollers, and the grade of gear. It is also effective to use a construction in which a conveyance amount coincides with an amount of rotation of the number of integer times of the motor or the gear. This makes a stop error of the motor and an eccentricity precision component of the gear cancel one another.
However, in conventional recording apparatus, although consideration has been given to the precision of a theoretical rotation amount (conveying plane movement amount) with respect to the conveying rollers for conveying the recording sheet, a countermeasure against restriction of the positions of the conveying rollers is insufficient. In the conveying roller which is arranged on the downstream side of the recording head (for example, what is called a discharge roller), particularly, a consideration and a countermeasure against such a point are insufficient. FIG. 8 is a cross sectional view showing a general conveying roller and its bearing in the conventional recording apparatus, taking an example of a discharge roller. In FIG. 8, there are a discharge roller 1001; a bearing 1002 of the discharge roller; and a driven roller 1003. The driven roller 1003 is pressed to the discharge roller 1001 by a force Fs by a spring (not shown) in order to produce a conveying force of the recording sheet.
A looseness of the discharge roller 1001 in the bearing 1002 causes it the roller to deviate in the downward direction in the diagram because of the pressing force Fs of the driven roller 1003. The cross sectional shape of both the discharge roller 1001 and the bearing 1002 is circular. Therefore, the apparatus has a construction in which play exists, and the discharge roller 1001 is easily moved in the directions shown by arrows Y and Y′ on an inner circumference of the bearing 1002. Consequently, if an external force due to a disturbance is applied, the discharge roller 1001 is liable to be moved and the position is difficult to be fixed. Since the position of the discharge roller is difficult to be fixed as mentioned above, it is difficult to maintain the high conveyance precision of the recording sheet and it is difficult to improve the image quality of the recording image.